June 2005
Allrounders for that special job. Difficult applications are child's play for air-operated diaphragm pumps
Air-operated diaphragm pumps are genuine all-rounders when the job is all about moving liquids safely. Special designs make even very unusual applications such as mixing paints, transferring two different mediums or pumping poisonous or abrasive substances possible.
Anyone who has to move liquids from drums, IBCs or other containers automatically thinks of centrifugal and barrel pumps. But all the circumstances must be right for these pump types to be effective. It is not always possible to ensure a continuous flow of liquid to the pump and so avoid dry-running. Moving abrasive liquids or pumping a medium through several storeys can often represent a serious challenge for centrifugal pumps. This is where the versatile air-operated diaphragm pump comes into its own - it is self-priming and can deal with liquids containing solids in suspension, abrasive mediums and a wide range of viscosities.
The pumps are driven by compressed air which makes expensive anti-explosion protection and explosion-proof electric motors superfluous. A further plus is the ease of control provided by the compressed air drive. Although centrifugal pumps will continue pumping to a limited degree against closed taps or valves, air-operated diaphragm pumps simply stop when the outlet is closed and restart immediately when the discharge valve is opened. In addition, the delivery rate can be very simply controlled and is infinitely variable merely by adjusting the compressed supply.
There is one further advantage which is inherent in the operating principle behind the pump and which is particularly important for sensitive liquids - the gentle process used by these pumps to move the liquids. The impellers of centrifugal pumps normally create significant shear forces but these forces are comparatively small in the displacement created by a pump diaphragm. This is why the paint and varnish industry often use air-operated diaphragm pumps to convey sensitive water-based paints and pigment-enriched solutions.
"The market for diaphragm pumps is huge - we are assuming a growth of 5 to 10 percent in diaphragm pumps " explained Heinz Hofmann, the Flux Pumps Sales Manager. A few years ago FLUX took over the air-operated diaphragm pumps previously made by Reko, thus extending their own range. FLUX engineers are now at work to increase the upper end of this range and a number of ATEX-certified models are already available.
A compact and solid construction and a wear-resistant air control system which needs no seals and can be driven by oil-free compressed air are standard features of the Reko-Flux pump. Five different sizes provide a series of pumps with graduated delivery rates up to 385 l/min and a maximum operating pressure of 8 bar. Several different plastics are used for the pump housing and different elastomers and plastics are available for the diaphragms depending on the application. All connections which come into contact with the medium are located on the side of the pump to prevent the inner pump housing coming into contact with the outer pump housing if there should be a leak either at the suction side or the discharge side. Undesirable pulsations, which are inevitable in all single-stage diaphragm pumps due to the operating principles involved, are reduced by 95 percent with a special pulsation damper.
Transfer two liquids at the same time
Eine Besonderheit der Druckluft-Membranpumpe sind die beiden Verdränger, die dazu genutzt werden können, gleichzeitig zwei untersiedliche Flüssigkeiten zu fördern oder anzusaugen. Eine spezielle Ausführung mit getrenntem Zu- und Ablauf machen´s möglich. "Ein Druckfarbenhersteller setzt diese Konstruktion beispielsweise dazu ein, um Farben, Pigmente oder Hilfsstoffe im Druckstutzen miteinander zu mischen", berichtet Hofmann aus der Praxis.
The two displacers which can be used to convey or extract two different liquids at the same time are a peculiarity of air-operated diaphragm pumps. A special model with a separate supply and delivery side make this possible. "A printing ink manufacturer, for example, uses this type of pump to mix dyes, pigments or process materials together at the outlet" states Hofmann, drawing on his experience.
A further application is the loading of filter presses using a special, compressed air-driven high pressure diaphragm pump system. The gentle method of operation is particularly important here as the flocculated sludge must not be destroyed. The system consists of two types of pumps - a normal air-operated diaphragm pump used as a loading pump and a high-pressure air-operated diaphragm pump. The system operates fully automatically - both pumps fill the filter chamber until a pressure of 6 to 7 bar is reached when the normal loading pump stops automatically and the high-pressure pump continues alone until 14 bar is reached. As the pressure in the filter increases, the pumps' delivery rate is automatically reduced.
A special model (RFMA) allows particularly abrasive media or media with large solids such as enamel slurry, crushed pumice or glaze for roof tiles to be conveyed. The pumps, which are powered directly by compressed air, have valves whose ball-seal insert is made from a specially wear-resistant polyurethane. When wear finally occurs, it is a simple job to replace these inserts.
Hermetic seal chamber provides additional seal
Type RFMD air-operated diaphragm pumps are used for the conveyance of hazardous liquids or media which must not come into contact with another substance, for example waste water containing radio-active elements or if heavy metals are involved.
These pumps are equipped with seal chambers which have a dual function if the diaphragm should split: on the one hand they prevent the medium coming into contact with the air in the pump and therefore ultimately being discharged into the atmosphere and, on the other hand, stop the medium from touching the metal components of the compressed air controls in case a chemical reaction is triggered. But it is still necessary to replace the diaphragm as quickly as possible if one should fail. A diaphragm monitoring system located in the seal chamber provides an optical or acoustic signal and cuts off the air supply to stop the pump if a diaphragm ruptures.
The medium which the pump has to convey is the determining factor in the choice of material for the pump. The seal chamber and the diaphragms which come into contact with the compressed air and the medium must be made from the same material as all will be affected by the medium if the diaphragm should split.
